This invention relates to a method for the control of dust during mining operations.
In the mining and processing of ores, ore handling points such as milling, crushing, stockpiling, dump pockets and conveyor belt transfer points give rise to significant airborne dust pollution problems. This is a particular problem with ores such as copper ores since many copper mines are located in arid regions of the world. There are increasing pressures to control dust for environmental reasons as well as for protecting the health of mine workers. Conventional mine dust suppression methods typically comprise spraying a solution of a dust suppressant composition onto surfaces that normally generate dust such as haul roads, etc. These compositions and methods are, however, often not satisfactory for use on ore at transfer points where the ore is dumped from one conveyor belt onto another or dumped from a conveyor belt onto a heap, since such dust suppressant compositions can interfere with further processing of the ore such as leaching and solvent extraction steps. This is a particular problem where bacteria are involved in heap leaching the ore. Dust control agents based on formulations containing conventional surfactants such as nonylphenol ethoxylates, alkylbenzene sulfonates, alcohol ethoxylates, etc., are toxic to these bacteria or are toxic to the bacteria when the quantities of the dust control agents are not closely monitored and controlled.
There is a strong need for dust control agents that can be freely used on ores during crushing, milling, conveying and stockpiling, i.e. dust control agents that do not interfere with the further processing of the ores, including the bioleaching of sulfide ores.
It has now been discovered that aqueous dust suppressing compositions (dust control agents) containing certain alkyl polyglucosides which exhibit fast wetting times and weak emulsifying properties result in minimal interference with the phase separation step of the solvent extraction process for the recovery of heavy metals from their ores such as the recovery of copper from copper ore using heap, dump, or vat leaching in combination with solvent extraction and electrowinning, and in addition exhibit very low toxicity to the bacteria responsible for leaching copper and other metals from sulfide ores. Hence, the aqueous dust suppressing compositions of the invention can be applied directly to ore crushing, milling and transport, especially on conveyor belts, preferably at conveyor belt transfer points, as highly effective dust control agents.
The alkyl polyglucosides used in the aqueous compositions of the invention have alkyl groups that contain an average of from 8 to 12 carbon atoms and a degree of polymerization of from 1.4 to 1.6. Such alkyl polyglucosides rapidly wet the solid material such as the ore and dust, etc., onto which they are applied, retard the rate of water evaporation from the applied solids, exhibit acceptable foaming levels and are environmentally acceptable in that they are readily biodegradable and exhibit low toxicity and favorable aquatic and eco-toxicity. The present invention accordingly relates to a process for suppressing dust on conveyor belts or at conveyor belt transfer points which comprises contacting the conveyor belt contents with a dust suppressing effective amount of an aqueous solution of an alkyl polyglucoside or alkyl polyglucoside mixture of the formula I
R1O(Z)axe2x80x83xe2x80x83I
wherein R1 is a monovalent alkyl radical having on average from 8 to 12 carbon atoms; Z is a glucose residue; a is a number having a value from 1.4 to 1.6.
The above alkyl polyglucosides and mixtures thereof also provide rapid phase disengagement, clean interface after phase separation, rapid kinetics of metal extraction, good selectivity for metals such as copper over iron extraction, low tendency for foaming, and low viscosity for ease of handling.
Other than in the operating examples, or where other indicated, all numbers expressing quantities of ingredients or reaction conditions used herein are to be understood as modified in all instances by the term xe2x80x9caboutxe2x80x9d.
While the present methods and compositions can be used with metal ores including metal sulfide ores other than copper ores, the following description will be directed to copper ores for which the invention will most likely have the greatest applicability.
Prior methods and compositions for dust suppression, as discussed above, are not all satisfactory for use in open pit copper oxide ore mining operations using solvent extraction for metal recovery because many of those surfactants used in the spray solutions interfere with the phase separation step. More specifically, in open pit copper mining operations wherein the ore is referred to as oxide ore, the copper is recovered from the ore by leaching with aqueous acid. The leaching is accomplished by trickling aqueous acid, such as dilute sulfuric acid, over and through pieces of ore having a relatively large surface area. The leach solution dissolves salts of copper and other metals as it trickles through the ore, to provide an aqueous solution of the mixture of metal values. The aqueous solution is mixed in tanks with an extraction reagent which is dissolved in an organic solvent, e.g., a hydrocarbon fraction. This is commonly referred to as the organic phase. The reagent includes an extractant chemical which selectively forms metal-extractant complex with the copper ions in preference to ions of other metals. The step of forming the complex is called the extraction or loading stage of the solvent extraction process. The organic phase is transferred to a large settling tank, where the organic solvent, now containing the copper-extractant complex in solution, is separated from the depleted aqueous solution (aqueous phase). This part of the process is called phase separation. Usually, the process of extraction is repeated through two or more mixer/settler stages, in order to more completely extract the desired metal. The process is described in detail in U.S. Pat. No. 4,582,689, the entire contents of which are incorporated herein by reference.
If the ore that is used is coated with conventional surfactants used for dust control and these surfactants enter the leach solution during the leaching step, phase separation does not occur cleanly when the leach solution is mixed with the organic phase during the solvent extraction process. That is, the surfactant acts as an emulsifier, causing the organic and aqueous phases to become suspended in each other thereby retarding the rate of phase separation and increasing entrainment values, thus adding cost to the entire process.
The compositions of the present invention cause minimal interference with the phase separation step, and this is true for sulfide ores as well as for oxidic ores and ores of other chemical composition.
With specific reference to copper sulfide ores and other metal sulfide ores, the sulfide ores are typically bioleached with iron and sulfur oxidizing bacteria, such as Thiobacillus ferrooxidans, Leptospirillum ferrooxidans, Thiobacillus thiooxidans, Sulfobacillus species (which naturally develop within bioheaps when the temperature rises above 40xc2x0 C.), and other microorganisms such as the extremely thermophilic Archaea microorganisms that include Sulfolobus, Acidianus, and Metallosphaera species. In general, some or all of these bacteria are endemic in the ore. In the bioleaching step, iron oxidizing bacteria oxidize ferrous iron to ferric iron which in turn oxidizes the sulfide ion to sulfur, resulting in liberation of the copper from the insoluble sulfide. It has now been discovered that the application of effective quantities of the aqueous dust suppressing compositions of the invention to the sulfide ores during crushing, milling or conveying, preferably at conveyor belt transfer points, does not result in toxic effects on the bacteria during the bioleaching step. In addition, close control over the quantities applied is generally not necessary since toxicity to the bacteria has not been found to be a serious problem.
The term xe2x80x9cconveyor belt transfer pointsxe2x80x9d includes the transfer of ore from one conveyor belt onto another, and the dumping of ore from a conveyor belt into a heap. The term xe2x80x9cdustxe2x80x9d used herein means any particulate solid material that is susceptible to suspension in air or other atmospheric environment. Accordingly, the term dust includes particles having an average diameter of up to 1 mm (though typically only up to about 300 micron) and down into the fume range (e.g. typically as low as 0.001 micrometers). The particulate friable solid will be in a substantially dry, non-fluid environment, e.g. the solid will be comprised of less than about 15% by weight water. The friable solids can have a varying chemical composition. Examples include stones, ore particles, organic materials e.g. sawdust, grain dust, fiber dust, or animal waste, and mineral particles, especial silicic or carbonaceous minerals, e.g. coal dust, soil dust (humus and/or subsoil), gravel dust, fine clay, lime dust, fly ash, and the like.
As discussed above, the alkyl polyglucosides or mixtures thereof that are useful in the process according to the invention are those of the formula I
R1O(Z)axe2x80x83xe2x80x83I
wherein R1 is a monovalent organic radical having an average of from 8 to 12 carbon atoms; Z is a glucose residue; a is a number having a value from 1.4 to 1.6. Such alkyl polyglucosides are commercially available, for example, as APG(copyright), GLUCOPON(copyright), PLANTAREN(copyright) or AGRIMUL(copyright) surfactants from Cognis Corporation, Ambler, Pa., 19002. Examples of such surfactants include but are not limited to:
1. GLUCOPON(copyright) 220 Surfactantxe2x80x94an alkyl polyglucoside in which the alkyl group contains 8 to 10 carbon atoms, an average alkyl group size of about 9.1 carbon atoms, and having an average degree of polymerization of 1.5.
2. APG(copyright) 325 Surfactantxe2x80x94an alkyl polyglucoside in which the alkyl group contains 9 to 11 carbon atoms and having an average degree of polymerization of 1.6.
3. GLUCOPON(copyright) 425 Surfactantxe2x80x94an alkyl polyglucoside in which the alkyl group contains 8 to 16 carbon atoms, an average alkyl group size of about 10.3 carbon atoms, and having an average degree of polymerization of 1.45. The GLUCOPON(copyright) 425N surfactant is identical to GLUCOPON(copyright) 425 except that it has been neutralized.
Related Cognis surfactants that are however, outside the scope of the invention, include the following:
1. GLUCOPON(copyright) 225 Surfactantxe2x80x94an alkyl polyglucoside in which the alkyl group contains 8 to 10 carbon atoms, an average alkyl group size of about 9.1 carbon atoms, and having an average degree of polymerization of 1.7.
2. GLUCOPON(copyright) 625 Surfactantxe2x80x94an alkyl polyglucoside in which the alkyl group contains 12 to 16 carbon atoms, an average alkyl group size of about 12.8 carbon atoms, and having an average degree of polymerization of 1.6.
Solutions of the alkyl polyglucosides are applied to the solid materials on the conveyor belts or at conveyor belt transfer points as described above. These aqueous solutions can be placed in contact with the friable solid material in any manner effective to suppress the generation of dust therefrom, such as by spraying.
The amount of the alkyl polyglucoside in the spray solution is an effective amount which is any amount effective to suppress the generation of dust from the friable solid onto which the solution is applied. Typically, the amount will be from about 5 ppm to about 1000 ppm of alkyl polyglucoside in water with the preferred amount being from about 10 ppm to about 100 ppm.
In addition to the above alkyl polyglucosides, these aqueous solutions can also contain a hydrophilic binder material, such as an acrylic latex or a polysaccharide.
The invention will be illustrated but not limited by the following examples.